Many companies are developing electronic paper, which is a display system that retains images with little or no power. Images typically are generated on an electronic paper medium by selectively applying an electric field to switchable display elements (e.g., dichroic spheres) in localized regions of the medium. In a typical implementation, an electrically conductive backplane electrode is placed behind the electronic paper medium and a second electrically conductive front plane electrode is placed in front of the electronic paper medium. Applying an electric field of one polarity to the medium switches the display elements to one orientation (e.g., black-side-up), and reversing the polarity of the applied electric field switches the display elements to a second orientation (e.g., white-side-up). A two-dimensional electrode grid with individually addressable cells may be used to provide an electric field in selected areas of the electronic paper medium. Alternatively, a single electrode or multiple electrodes in a head may be scanned across the electronic paper as the paper is advanced by a roller system. In some systems, the position of the electronic paper is fixed and one or more electrodes are scanned across the electronic paper. The electronic paper medium remains in the switched (or “printed”) state after the electric field is removed, until a new electric field is applied to change the orientation of the display elements.
One known electrode array printer for printing on rewritable electronic paper includes an array of independently addressable electrodes, each capable of applying a localized field to the rewritable media to rotate dichroic spheres within a given pixel area of a rewritable medium. In another known electrically writable media printing technique, a laser scanner is used to erase a uniform high-voltage charge that was deposited on the surface of a photoconductor drum or belt. The voltage swing between charged and discharged areas of the photoconductor is conventionally on the order of about 500–600 volts. When the rewritable medium is brought in contact with the charge-written photoconductor through a biased back electrode roller, electric fields that are generated between the photoconductor and back electrode cause color rotation of the dichroic spheres to develop a desired print image.